In a typical cellular network, also referred to as a wireless communication system, User equipment, UEs, communicate via a Radio Access Network, RAN, to one or more core networks, CNs.
A user equipment is a mobile terminal by which a subscriber may access services offered by an operator's core network and services outside operator's network to which the operator's RAN and CN provide access. The user equipment may be for example communication devices such as mobile telephones, cellular telephones, smart phones, tablet computers or laptops with wireless capability. The user equipment may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipment are enabled to communicate wirelessly in the cellular network. The communication may be performed e.g. between two user equipment, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station, RBS, which in some radio access networks is also called eNodeB (eNB), NodeB, B node or network node. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment within range of the base stations.
Multiple Input Multiple Output, MIMO, refers to any communications system with multiple antennas at the transmitter and receiver, and it is used to improve communication performance. The terms input and output refer to the radio channel carrying the signal, not to the devices having antennas. At the transmitter, Tx, multiple antennas may be used to mitigate the effects of fading via transmit diversity and to increase throughput via spatial division multiple access. At the receiver, Rx, multiple antennas may be used for receiver combining which provides diversity and combining gains. If multiple antennas are available at both the transmitter and receiver, then different data streams may be transmitted from each antenna with each data stream carrying different information but using the same frequency resources. For example, using two transmit antennas, one may transmit two separate data streams. At the receiver, multiple antennas are required to demodulate the data streams based on their spatial characteristics. In general, the minimum number of receiver antennas required is equal to the number of separate data streams. 4×4 MIMO, also referred to as four branch MIMO, may support up to four data streams.
Several new features are added for the long term HSPA evolution in order to meet the requirements set by the International Mobile Telecommunications-Advanced, IMT-A. The main objective of these new features is to increase the average spectral efficiency. One possible technique for improving downlink spectral efficiency would be to introduce support for four branch MIMO, i.e. utilize up to four transmit and receive antennas to enhance the spatial multiplexing gains and to offer improved beamforming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high Signal to Noise Ratio, SNR, user equipment and improves the coverage for low SNR user equipment.
Channel feedback information enables a scheduler to decide which user equipment should be served in parallel. The user equipment is configured to send three types of channel feedback information: CQI, RI and PMI.
CQI is an important part of channel information feedback. The CQI provides the base station with information about link adaptation parameters which the user equipment supports at the time. The CQI is utilized to determine the coding rate and modulation alphabet, as well as the number of spatially multiplexed data streams.
RI is the user equipment recommendation for the number of layers, i.e. streams to be used in spatial multiplexing. RI is only reported when the user equipment operates in MIMO mode with spatial multiplexing. The RI may have the values 1 or 2 in a 2×2 MIMO configuration and it may have the values from 1 and up to 4 in a 4×4 MIMO configuration. The RI is associated with a CQI report. This means that the CQI is calculated assuming a particular RI value. The RI typically varies more slowly than the CQI.
PMI provides information about a preferred pre-coding matrix in a codebook based pre-coding. PMI is only reported when the user equipment operates in MIMO. The number of pre-coding matrices in the codebook is dependent on the number of antenna ports on the base station. For example, four antenna ports enables up to 64 matrices dependent on the RI and the UE capability. A Precoding Control Indicator, PCI, indicates a specific pre-coding vector that is applied to the transmit signal at the base station.
The Multiple Input Multiple Output, MIMO, is an advanced antenna technique to improve the spectral efficiency and thereby boosting the overall system capacity. The MIMO technique may use a commonly known notation (M×N) to represent MIMO configuration in terms number of transmit (M) and receive antennas (N). The common MIMO configurations used or currently discussed for various technologies are: (2×1), (1×2), (2×2), (4×2), (8×2) and (8×4). The configurations represented by (2×1) and (1×2) are special cases of MIMO and they correspond to transmit diversity and receiver diversity, respectively. The configuration (2×2) is used in and supported by WCDMA release 7, and the configurations ((4×4), (4×2), (4×1)) are being defined in WCDMA release 11.
A 4Tx transmissions scheme for HSDPA is currently discussed within 3GPP standardization, while previous versions of the specification supports up to 2Tx antenna transmissions. In order to support 4Tx MIMO transmissions, it is necessary to obtain 4 channel estimates in order to characterize each of the spatial layers, which means that more pilot signals will be necessary. Common pilot signals are used for two main functionalities: Channel State Information, CSI, estimation through channel sounding whereby RI, CQI and PCI may be estimated, and channel estimation for data demodulation purposes.
For a 4-branch MIMO, two different approaches may be used: common pilot signals for both CSI and channel estimation for data demodulation; and common pilot signals for CSI estimation and additional high power pilot signals for channel estimation for data demodulation. In the above context, “common pilots” or “common pilot signals” refer to pilot signals that are made available to all user equipments and which are transmitted without user specific beamforming.
Common pilot signals, or common pilots, may be transmitted at instances in which legacy user equipments, e.g. Release 7 MIMO and Release 99, who are not able to demodulate 4Tx transmissions are scheduled. These legacy user equipments cannot make use of the energy in the common pilot signals. However, the energy in the additional common pilot signals will reduce the amount of energy available for HS-PDSCH scheduling to the legacy user equipments.
Moreover, the additional pilot signals cause interference to these user equipments. Therefore, to minimize performance impacts to non-4Tx user equipments, it is essential that the power of the common pilot signals can be reduced to a low value.
Unfortunately with reduced pilot power of common pilot signals, the demodulation performance will be impacted. Hence, two additional pilot signals with higher power are introduced for data demodulation in a four branch MIMO system. These additional pilot signals are sometimes called scheduled pilot signals or demodulation common pilot signals. A base station may configure these additional pilot signals based on channel conditions and available power.
Hence, the pilot signal design schemes for 4-branch MIMO may be divided in to:                A. Common pilots for CSI estimation and data demodulation        B. Common pilots for CSI estimation and additional pilots for data demodulationA. Common Pilots for CSI Estimation and Data Demodulation        
FIG. 1 shows a schematic block diagram illustrating an example of a telecommunications system 150 with common pilots for CSI estimation and data demodulation. In other words, the figure shows a conceptual diagram of an example of a common pilot design system. Here, it is assumed that the transmitter Tx is assumed to be a transmitter of a network node and the receiver Rx a receiver of a user equipment.
As can be seen in FIG. 1, the network node transmitter, Tx 151, may transmit known pilot symbols, i.e. common pilots 153, e.g. CPICH, for channel estimation for channel sounding. When data is present, the data 154 is precoded, e.g. via precoder 155, and then transmitted together with the common pilots 153. The transmitted data 154 and common pilot signals 153 are received by the user equipment receiver, Rx 152.
The transmitted data 154 may be detected by the user equipment, e.g. via a data detector 156. The user equipment may also estimate channel quality, typically SINR, from the channel sounding, e.g. via a channel estimator 157. The user equipment may also compute the preferred precoding matrix and CQI for the next downlink transmission, e.g. via a pre-coder matrix calculator 158. This information may be conveyed to the network node through a feedback channel 159, e.g. HS-DPCCH.
The network node may process this information, i.e. the feedback information from the user equipment, and determine the precoding matrix, modulation, coding rate, and some other parameters, such as transport block size, etc. The network node may convey this information to the user equipment through a downlink control channel. The network node then transmits data with the modulation and coding rate indicated in the downlink control channel. The network node may pre-multiply the data by a precoding vector/matrix before passing the data to the antenna ports. The user equipment may estimate the channel for data demodulation from the common pilot symbols, which is shown in FIG. 1.
It should be noted that a solution based on common pilot signals only may have a negative impact on legacy user equipments unless the power on the additional pilot signals is minimal.
FIG. 2 shows a diagram illustrating system throughput with different pilot powers in a telecommunication system. In other words, the figure shows the performance of a pilot reduction scheme on the sector throughput with different number of user equipments per sector.
Here, it is assumed that all the user equipments are Release-7 MIMO capable with 2 receive antennas, 2Rx. The pilot signal powers for the first and the second antennas are set to −10 dB and −13 dB, respectively. The additional interference due to third (3rd) and fourth (4th) pilot signals is considered with different power levels of the pilot signals, i.e. at −25 dB (2a), −22 dB (2b), −19 dB (2c), −16 dB (2d), −13 dB (2e). These are compared to the 2×2 MIMO case with no interference (2), i.e. no interference by a third or fourth antenna.
Here, it may be observed that as the power of the additional pilot signals is decreased, the impact on the system throughput performance is less. For example, if the pilot signal power is around −19 dB (2c), the impact on the legacy user equipments may be considered almost negligible. However, if the pilot signal power is minimal, e.g. around −13 dB (2e), then the demodulation performance of 4Tx user equipments, i.e. non-legacy user equipments, will be adversely impacted.
FIG. 3-4 shows diagrams illustrating link level performance when common pilot signals are used for CSI estimation and data demodulation in a telecommunication system.
Common pilot signals may be transmitted at instances in which legacy UEs (Release 7 MIMO and Release 99) that are not able to demodulate 4Tx transmissions, are scheduled. These legacy UEs cannot make use of the energy in the 3rd and 4th common pilot signals. Also, the energy made available in the 3rd and 4th pilot signals reduces the amount of energy available for HS-PDSCH scheduling to the legacy UEs. Moreover, the 3rd and 4th common pilots may cause interference to these UEs which at best may make use of the 1st and 2nd common pilot signals. Therefore, to minimize performance impacts to non-4Tx UEs, it is desirable that the power of at least the 3rd and 4th common pilot signals be reduced to a low value.
A solution based only on common pilots will have a negative impact on the legacy UEs unless the powers on the 3rd and 4th common pilots are minimal. However, if the powers are minimal, then the demodulation performance of 4Tx UEs will be adversely impacted.
FIGS. 3 and 4 show example link level throughputs as a function of pilot powers on 3rd and 4th pilots for a non-legacy UE with three different geometries for 4×4 MIMO and 4×2 MIMO systems, respectively. In these figures, the pilot powers for the 1st and 2nd pilots are maintained at −10 and −13 dB, respectively.
It can be observed that as the 3rd and 4th pilot powers are reduced, the performance of the non-legacy UE degrades due to bad channel estimation for CQI and data demodulation. The degradation is severe at a high C/I, e.g. at 20 dB (3c; 4c). This is because at high C/I, there is a high probability of rank 3 and rank 4 transmissions and/or high data rates, which require a larger amount of pilot power energy. On the other hand, low data rates and/or rank selections, which occur at low C/I, e.g. at 0-10 dB (3a, 3b; 4a, 4b) can be demodulated with a lower amount of pilot energy, i.e. a higher traffic-to-pilot ratio.
Introduction of additional pilot signals, when any 4-branch MIMO user equipment is scheduled, may cost some additional overhead and may not give benefit for all the scenarios. In reality, a high amount of pilot signal power is required when the UE is attempting to demodulate high data rates with high rank.
B. Common Pilots for CSI Estimation and Additional Pilots for Data Demodulation
FIG. 5 shows a schematic block diagram illustrating an example of a telecommunications system or system scheme with common pilots and additional scheduled/demodulation pilots.
Similar to the common pilot signal scheme as shown in FIG. 1, the network node transmitter, Tx 151, may transmit known pilot symbols, i.e. common pilots 153 (e.g. CPICH), for channel estimation for channel sounding. Data 154 is precoded, e.g. via precoder 155, and then transmitted together with the common pilots 153. The transmitted data 154 and common pilot signals 153 are received by the user equipment receiver, Rx 152.
The transmitted data 154 is detected by the user equipment, e.g. via a data detector 156. The user equipment may also estimate channel quality, typically SINR, from the channel sounding, e.g. via a channel estimator 157. The user equipment may also compute the preferred precoding matrix and CQI for the next downlink transmission, e.g. via a pre-coder matrix calculator 158. This information may be conveyed to the network node through a feedback channel 159, e.g. HS-DPCCH.
For downlink data transmission, the network node uses this information and chooses the precoding matrix, modulation, coding rate, CQI and the transport block size. The network node may convey this information to the user equipment through a downlink control channel. The network node then transmits data with the modulation and coding rate indicated in the downlink control channel. The network node may pre-multiply the data by a precoding vector/matrix selected by the network node before passing the data to the antenna ports.
In addition to the data 154, additional pilot signals 160 similar to common pilot signals 153 without precoding are transmitted with high power from all or few subset of antennas, e.g. for the 3rd and 4th antennas. These additional pilots 160 may be referred to as scheduled pilot signals or demodulation pilot signals. The user equipment may estimate the channel for the data demodulation from the demodulation pilot signals or the common pilot symbols.